1. Field of the Invention
The present invention relates to the preparation of carboxylic acid anhydrides, and, more especially, to the preparation of carboxylic acid anhydrides, notably acetic anhydride, by carbonylation of a carboxylic acid ester.
2. Description of the Prior Art
It is well known to this art that, e.g., acetic anhydride can be prepared by carbonylation of methyl acetate, under relatively severe conditions of pressure, in the presence of nickel complexes of the formula EQU [A.sub.4 M].sub.2 NiX.sub.4
in which X represents a bromine or iodine atom, M represents a phosphorus or nitrogen atom and A is, for example, a lower alkyl radical (compare U.S. Pat. No. 2,729,651). These complexes, which are obtained by reacting nickel halides with quaternary phosphonium or ammonium halides, can be employed in this form in the subject reaction, or, alternatively, they can be formed in situ. However, the efficiency of this type of process is low, despite the high pressures used.
More recently, catalyst systems have been proposed which make it possible to carbonylate methyl acetate under less severe pressure conditions. Thus, U.S. Pat. No. 4,002,678 describes the carbonylation of methyl acetate in the presence of nickel, chromium, methyl iodide and a phosphine (or an amine), under a pressure of less than 70 bars.
Parallel thereto, it has also been shown that chromium is not necessary in a process of the aforesaid type if the reaction is carried out in the presence of carboxylic acid solvent (compare published French Patent Application No. 2,408,571). Nevertheless, the industrial-scale development of these recent techniques, the value of which is not disputed in principle, is jeopardized on the one hand by the instability and the cost of the phosphines or amines required to carry them out, and on the other hand by the relatively low efficiency of the catalyst systems in question.